fix: reverse logic in brew-bump env fns

[jsjoeio]

This PR uses is_env_var_set correctly.

Fixes #4686

[codecov]

Codecov Report

Merging #4732 (70e70db) into main (15500bb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4732   +/-   ##
=======================================
  Coverage   69.23%   69.23%           
=======================================
  Files          29       29           
  Lines        1638     1638           
  Branches      341      341           
=======================================
  Hits         1134     1134           
  Misses        428      428           
  Partials       76       76           

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[github-actions]

✨ Coder.com for PR #4732 deployed! It will be updated on every commit.

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